In HTML and XHTML, an image map is a list of coordinates relating to a specific image, created in order to hyperlink areas of the image to various destinations. This differs from a normal image link, where the entire area of the image links to a single destination. For example, a map of the world may have each country hyperlinked to further information about that country. The intention of an image map is to provide an easy way of linking various parts of an image without dividing the image into separate image files.
For instance, a normal image that happens to contain brand name products, could be turned into an image map which would redirect the user who clicked on a branded item to the landing page of that advertiser.
For additional information please refer to this article on Wikipedia.org.